Various constructions are known for air compressors used, for example, for driving pneumatic tools, paint spray guns, air dusters, and the like. In one type of compressor, a piston is mounted to reciprocate in a cylinder. The piston is connected through a connecting rod to an eccentrically rotated crank which causes the piston to reciprocate. The piston may be cylindrically shaped and confined to slide in the cylinder, in which case a wrist pin provides a pivotal connection between the piston and the connecting rod. This construction requires constant oil lubrication during operation. Or, the piston may be rigidly secured to the connecting rod and designed to rock or tilt as it is reciprocated in the cylinder, as is shown, for example, in U.S. Pat. Nos. 3,961,869, 4,028,015, 4,540,352 and 4,848,213. A resilient seal is secured to the piston to allow it to slide and to tilt while maintaining a gas tight seal between the piston and the cylinder. Since the seal does not require constant lubrication, this construction is sometimes referred to as an "oilless" compressor. The operating life of the seal is related to the maximum temperature to which the seal is subjected during operation.
Most compressors are driven at relatively low speeds by synchronous alternating current electric motors. For example, a motor operated from 60 Hz. power typically operates at about 1,700 rpm and the compressor speed may be further reduced by a belt and pulley drive. A synchronous motor may have difficulty in starting a loaded compressor. The reciprocating piston and other moving components for oil lubricated compressors are relatively massive and not suitable for operation at high speeds, for example, as are achieved by non-synchronous universal motors. It is believed that higher operating speed universal motors have not been used for driving larger oilless compressors because the heat produced by the motor can significantly reduce the life of compressor components such as the sliding piston seal and of a drive belt and the noise caused by a high speed gear reduction system is objectionable.